SF₆ can be a tricky gas to deal with. So, to understand it better:
Wikipedia: Sulfur Hexafluoride
EPA.gov: Sulfur Hexafluoride Basics
NIH.gov: Sulfur Hexafluoride | F6S | CID 17358
CDC.gov: NIOSH Pocket Guide to Chemical Hazards
More information on the underlying math can be found at:
A really good Schneider Electric paper
A really good Baker & Adamson paper
A brief discussion of the Beattie-Bridgeman equation